1. Field of Invention
The present invention relates to a tail-less light emitting diode (LED) control circuit; particularly, it relates to a tail-less LED control circuit which connects and disconnects an output capacitor in synchronization with a pulse width modulation (PWM) dimming signal, to alleviate LED afterglow (tail).
2. Description of Related Art
FIG. 1 shows a schematic circuit diagram of a conventional LED control circuit. As shown in FIG. 1, an LED driver circuit 10 controls the inductor current in a power supply stage 60 via a switching signal pin SW, such that the power supply stage 60 provides electrical power to an LED circuit 30. The LED driver circuit 10 receives a PWM dimming signal 20; depending on different designs, such PWM dimming signal 20 may be received via an enable pin EN, a digital control pin DCTL, or an analog control pin ACTL. The LED driver circuit 10 turns ON/OFF the LED circuit 30 according to the PWM dimming signal 20, whereby the brightness of the LED circuit 30 is adjusted according to the duty ratio of the PWM dimming signal 20.
The aforementioned prior art has the following drawbacks. When the LED circuit 30 is turned OFF, due to the residual charges in the output capacitor Cout, the LEDs are still powered for a short while; that is, the LEDs will still shine for a short while after they have been turned OFF, which is an undesired “afterglow” or “tail”. And since the output capacitor Cout is discharged during the OFF period, it will need to be re-charged to the appropriate voltage level in the next cycle, requiring more settling time. Thus, the actual brightness of the LED circuit 30 does not exactly follow the duty ratio of the PWM dimming signal 20, resulting in low dimming contrast and reducing the effectiveness of the PWM dimming signal 20.
In view of the foregoing, the present invention provides a tail-less LED control circuit which connects and disconnects the output capacitor in synchronization with a pulse width modulation (PWM) dimming signal, to alleviating the foregoing problems. The present invention does not need a complicated circuit, so there is no area penalty.